This invention relates to thermal ink jet printers, and, more particularly, to control of the temperature of the print head ejectors of such printers during printing operations.
Printers are devices that print characters onto a printing medium such as a sheet of paper or a polyester film. Printers of many types are available, and are commonly controlled by a computer that supplies the images, in the form of text or figures, that are to be printed.
Some printers used a colored liquid, such as an ink or a dye, but generally termed herein a colorant, to form the images on the printing medium. (By contrast, other printers use a dry toner to form the image.) Such printers deliver the colorant to the medium using a print head that creates the proper patterning of colorant to record the image.
One important type of printer is the thermal ink jet printer, which forms small droplets of colorant that are ejected toward the printing medium in the pattern of dots. The droplets are formed when an electrical current is passed through an electrical resistor in the ejector, vaporizing a small volume of colorant. The vaporized colorant expands, driving a droplet of colorant out of a nozzle to deposit as a dot on the printing medium. When viewed at a distance, the collection of dots form the image, in much the same manner that images are formed in newspapers. Ink jet printers are fast, producing a high output of printed image, and quiet, because there is no mechanical impact during formation of the image except for the droplets of ink striking the printing medium.
Typically, a thermal ink jet printer has an ejector with a large number of individual colorant ejection nozzles in a print head, with one resistor for each nozzle, supported in a carriage and oriented in a facing, but spaced-apart, relationship to the printing medium. The carriage and supported print head traverse relative to locations on the surface of the medium, with the nozzles ejecting droplets of colorant, at appropriate times under command of the controller, to produce a swath of droplets. The droplets strike the medium and then dry to form "dots" of color that, when viewed together, form one swath of the permanently printed image. The carriage is then moved an increment in the direction normal to the traverse (or, alternatively, the printing medium is advanced), and the carriage again traverses the page with the print head droplet ejector operating to deposit another swath of dots. In this manner, the entire pattern of dots that form the image is progressively deposited by the print head during a number of traverses of the page. To achieve the maximum output rate, the printing is preferably bidirectional, with the print head ejecting colorant during traverses from left-to-right and right-to-left.
One of the key operating parameters of the print head and ejector is its temperature of operation. Thermal energy is generated with each operation of an ejection resistor. Some of the energy leaves the printer in the ejected droplet, but some remains in the print head to heat it. The print head is constantly cooled by conduction to the surrounding air. The actual temperature of the print head is the result of a balancing of heating and cooling of the print head.
A typical thermal ink jet printer has specified minimum and maximum operating temperatures of the ejector, that define its operating range. If the operating temperature is less than the minimum, the ejection resistors cannot impart enough energy to each droplet to achieve proper ejection. If the operating temperature is greater than the maximum, there may be spurious ejection, irregularities in the ejected droplets, and choking of the nozzles as gas dissolved in the ink leaves solution to form bubbles in the ink flow channels.
These minimum and maximum values are temperatures measured at the ejector of the print head, and do not correspond directly to the air temperature where the printer is operated. However, the air temperature plays a part in determining whether the printer can stay within the specified temperature range. That is, a cold air temperature tends to cause the ejector to be nearer the low end of its range, and a warm air temperature tends to cause the ejector to be nearer the high end of its range. To be a viable commercial product, the thermal ink jet printer must be able to operate over a range of air temperatures, and still maintain the ejector temperature within the acceptable range.
It is known to use heaters and fan coolers within the printer, operating under control of a temperature sensor, to assist in maintaining the temperature of the ejector within the proper operating range. See, for example, U.S. Pat. No. 4,704,620, which emphasizes that the temperature control of the print heads must be carefully controlled, and provides a method for ensuring that the heaters will not overheat the print head and that the fans will not overcool the print head. The approach described therein utilizes a calculation of the heat transfer coefficients of the heaters and the fan in an attempt to keep the heat flowing into or out of the ejector within preconceived limits that will result in maintenance of the temperature range. The ejector itself is small and has very low thermal mass, and therefore careful attention is required to avoid overheating or overcooling. The use of heaters and a fan encourages increasing the thermal mass to avoid temperature swings through and out of the acceptable operating range, but the general principles of print head design call for reduced mass that must be supported on and moved by the carriage.
Although the system described in the `620 patent and available in the art is presumably operable, there is a need for an improved thermal control system for a thermal ink jet printer. Such a control system would preferably not use a fan to cool the ejector, since this component adds cost and weight to the printer, and increases the chances of a breakdown. The control system would also preferably achieve more precise temperature control than possible using heaters and a fan, without increasing the thermal mass of the ejector. The present invention fulfills this need, and further provides related advantages.